1. Field of the Invention
The present invention relates to a high tenacity, high modulus and low shrinkage lyocell multifilament yarn suitable for industrial fibers, and preferably fibers for tire cords or MRG. More particularly, the invention relates to the production of a lyocell filament suitable for tire cords or MRG (mechanical rubber goods), in which the lyocell filament is produced by dissolving cellulose having a degree of polymerization (DPW) of 700-2,000, and a α-cellulose content of more than 90% in N-methylmorpholine N-oxide (hereinafter, referred to as NMMO)/water and spinning the cellulose solution by a dry-wet spinning process.
2. Background of the Related Art
As a frame forming the inner part of a tire, a tire cord is used at large amounts, and regarded as an important factor in the maintenance of a tire shape or ride comfort. Various tire cord materials, such as polyester, nylon, aramid, rayon and steel, are currently used, but they do not satisfy perfectly various functions required in the tire cord. Basic performances required in such tire cord materials are as follows: (1) high tenacity and high initial modulus; (2) thermal resistance, and non-deterioration in dry and wet heat; (3) fatigue resistance; (4) dimensional stability; (5) excellent adhesion to rubber, and the like. Thus, the tire cord materials are used in applications according to their intrinsic physical properties.
Among them, the greatest advantages of the rayon tire cord are its excellent thermal resistance and dimensional stability, and its elastic coefficient is maintained even at high temperature. Owing to such low shrinkage and excellent dimensional stability, it has been mainly used in high-speed radial tires for automobiles and the like. However, the rayon tire cord produced according to prior art has disadvantages in that tenacity and modulus are low, and a reduction in tenacity is shown due to its chemical and physical structures easy to absorb moisture.
Many kinds of fiber materials are used as a reinforcement material for hoses with gas or liquid conveyance and transport functions, conveyer belts with solid conveyance function, and transmission belts with power transfer function. Performance requirements of such materials include excellent tenacity, elasticity, creep, adhesion, thermal resistance, bending resistance and impact resistance.
Of them, when a rayon fiber is used as the reinforcement material, it have advantages in that it has little or no deformation even at high temperature and shows superior adhesion and bending properties. However, due to low elasticity and high elongation, it is disadvantageous in that it can be deformed when it undergoes a long-term or high load.
The prior industrial rayon which is used for tire cords or MRG is a fiber which is produced by the same wet spinning process as in viscose rayon except that several spinning conditions are changed to increase the tenacity of the fiber. Namely, it is produced by a method of increasing degree of orientation by increasing draw ratio in a drawing step after coagulation, and a method of increasing the amount of adding of ZnSO4 in a coagulation step to increase crystallinity while increasing a fiber skin layer (i.e., skin portion) to increase tenacity. However, to produce this fiber, cellulose is reacted with carbon disulfide to produce celluose-xanthate and dissolved in dilute sodium hydroxide solution to make a cellulose spinning solution. The spinning solution is spun in an aqueous sulfuric acid solution to produce a fiber. Thus, this production method has disadvantages in that a process is lengthened, large amounts of chemicals are used, and highly inflammable poisonous chemicals, such as carbon disulfide, must be handled. Also, it has an environmental problem in that hydrogen sulfide gas causing a nervous system disorder is generated during the production process.
In U.S. Pat. No. 5,942,327 which discloses the lyocell fibers produced according to prior art, cellulose was swollen in about 78 wt % of NMMO and then subjected to a distillation process to produce a cellulose solution. This patent showed that the tenacity and elongation of the cellulose filaments manufactured from this cellulose solution varied depending on air gap temperature, nozzle orifice diameter, and the temperature of a coagulation bath containing additives such as NH4Cl or CaCl2. In this case, the nozzle orifice diameter was changed to 130 μm and 200 μm. However, the manufactured cellulose filaments have a high elongation of about 9.0-13.0% due to low draw ratio, but they are disadvantageous in that they have a tenacity lower than 6.0 g/d, which makes it difficult to provide a lyocell fiber having superior physical properties to the prior rayon fiber for tire cords and MRG.